Deproteinized natural rubber latex, method of preparing the same, rubber product using the same, and proteolytic agent for deproteinized natural rubber latex

ABSTRACT

The present invention provides a deproteinized natural rubber latex wherein coagulation of a rubber component does not occur when the concentration of calcium ions (Ca 2+ ) is 0.01 mol/L or less and coagulation of the rubber component occurs when the concentration of Ca 2+  is 0.1 mol/L or more; a method of preparing the deproteinized natural rubber latex, which comprises adding a protease and two or more surfactants having different coagulation properties to calcium ions (Ca 2+ ) to a natural rubber latex and maturing the natural rubber latex; a rubber product using the deproteinized natural rubber latex; and a proteolytic agent comprising a protease and two or more surfactants. The deproteinized natural rubber latex is a latex wherein rubber molecules are dispersed and stabilized by a surfactant, and also have good film forming properties by means of the anode coagulation method and is less likely to cause uneven thickness of the film and liquid dripping even when a mold is dipped repeatedly in a latex on formation of a rubber film.

BACKGROUND OF THE INVENTION

The present invention relates to a deproteinized natural rubber latex,which is superior in balance between the film forming properties bymeans of the anode coagulation method and the dispersion stability of alatex, a method of preparing the same, a rubber product using thedeproteinized natural rubber latex, and a proteolytic agent for naturalrubber latex.

Natural rubbers have widely been used in various fields, for example,rubber gloves because of features such as large extension, highelasticity and strong film strength.

In the production of a glove made of a natural rubber, a productionmethod is employed according to the thickness of a rubber film. A rubberglove having a film thickness of about 1 mm, for example, glove for homeuse is generally produced by a so-called anode coagulation method ofdipping a mold (hand mold) for glove, the surface of which is previouslycoated with a coagulant (anode coagulant), in a natural rubber latex.

It has recently been required for a rubber product using a naturalrubber latex to highly remove a protein contained in the product. Mainreasons include (1) immediate (I type) allergy such as dyspnea orurticaria is caused by bringing a natural rubber product into contactwith the skin or mucosa and a protein contained in a natural rubberlatex is considered to be a causative agent; (2) the protein can causevariations in quality and vulcanization properties of the natural rubberproduct because the kind and quantity of the protein vary depending onthe locality and production season of the latex; and (3) the protein cancause deterioration of mechanical characteristics such as creepcharacteristics and aging resistance and electrical characteristics suchas insulating properties of the rubber product.

Japanese Published Unexamined Patent (Kokai Tokkyo Koho Hei) No. 6-56902discloses a method of removing a protein and a decomposition productthereof through a series of the steps of adding a proteolytic enzyme(protease) and a surfactant to a natural rubber latex, maturing thenatural rubber latex, thereby decomposing a protein in the latex, andsubjecting the latex to a centrifugation treatment. When subjected to adeproteinization treatment according to this method, the protein in thenatural rubber latex can be removed in a very high level and thenitrogen content (N %) as measured by the Kjeldahl method is reduced to0.1% by weight or less.

For the purpose of preventing coagulation of a rubber component causedby an operation such as stirring by stabilizing a latex unstabilized asa result of removal of a protein, a surfactant is incorporated into aso-called deproteinized natural rubber latex obtained by the methoddisclosed in the publication described above.

The surfactant not only improves the mechanical stability of the latex,but also exerts a large influence on the sensitivity to an anodecoagulant. When using a higher alcohol sulfate ester salt anionicsurfactant as the surfactant, the sensitivity of the latex to the anodecoagulant increases. Therefore, a film can be formed by the anodecoagulation method even under the same conditions as those in case ofthe non-deproteinized natural rubber latex.

However, because of too large sensitivity to the anode coagulant, thefilm is rapidly dried after dipping in the latex as compared with thecase of using the natural rubber latex. As a result, when a mold isrepeatedly dipped in the latex for the purpose of increasing thethickness of rubber film, there arise new problems such as uneventhickness of the rubber and liquid dripping.

When using a higher alkyl phenyl ether sulfate ester salt anionicsurfactant as the surfactant, the sensitivity of the latex to the anodecoagulant decreases and the rubber component is less likely to becoagulated. To form a film having nearly the same thickness as that incase of the non-deproteinized natural rubber by the anode coagulationmethod, there arise new problems that very special film formingconditions or a very complicated step are required.

The present applicants have previously found such a fact that a dipproduct having a sufficient film thickness can be obtained by using aspecific heat sensitizer and a specific anode coagulant in a specificcombination and incorporating the specific combination in a large amountas compared with a conventional formulation (Japanese PublishedUnexamined Patent (Kokai Tokkyo Koho) No. 2000-17002).

However, there were problems, according to the method described in thepublication described above, since the both of the heat sensitizer andthe anode coagulant are incorporated into the latex, the latex becomesunstable as compared with a conventional heat sensitizing method using anatural rubber latex, thereby making it impossible to obtain long-termstability and making it hard to control heat-sensitive properties.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a deproteinizednatural rubber latex, which is a latex whose rubber particles aredispersed and stabilized by a surfactant by means of a deproteinizationtreatment, and also which has good film forming properties by means ofthe anode coagulation method and is capable of producing a rubberproduct having a sufficient film thickness under the same film formingconditions as those of the prior art without causing uneven thickness ofthe film and liquid dripping even when a mold is dipped repeatedly in alatex on formation of a rubber film, and a method of preparing the same.

Another object of the present invention is to provide a dip productusing the deproteinized natural rubber latex by means of the anodecoagulation method.

A still another object of the present invention is to provide aproteolytic agent, which is capable of subjecting a natural rubber latexto a high-level proteolyzation or deproteinization treatment and canalso impart sufficient coagulation properties by means of the anodecoagulation method and good film forming properties without causinguneven thickness and liquid dripping to the proteolyzed natural rubberlatex or deproteinized natural rubber latex obtained by the treatment,and also which can improve balance with the dispersion stability of thelatex.

As described above, a conventional deproteinized natural rubber latexhad such problems that too high sensitivity to the anode coagulantcaused uneven thickness of the rubber film and liquid dripping in caseof dipping repeatedly, or that too low sensitivity to the anodecoagulant made it difficult to coagulate the rubber component.

However, the present inventors have studied intensively to solve theproblems described above and found the following quite new fact. Thatis, even if the coagulation properties to calcium ions (hereinafterreferred to as “Ca²⁺”) of the deproteinized natural rubber latex arecontrolled and the latex is added dropwise in an aqueous solution havingthe concentration of Ca²⁺ of 0.01 mol/L or less, coagulation of therubber component in the latex does not occur. However, when thecoagulation properties are controlled so that coagulation of the rubbercomponent occurs when the latex is added dropwise in an aqueous solutionhaving the concentration of Ca²⁺ of 0.1 mol/L or more, the sensitivityto anode coagulants such as calcium nitrate, calcium chloride and thelike becomes proper surprisingly, there by making it possible tomarkedly improve the film forming properties by means of the anodecoagulation method and to afford good balance with the dispersionstability of the latex. Thus, the present invention has been completed.

The present invention is directed to:

(I) a deproteinized natural rubber latex which is obtained by subjectingto a treatment for decomposition and removal of a protein, whereincoagulation of a rubber component does not occur when the concentrationof calcium ions (Ca²⁺) is 0.01 mol/L or less and coagulation of therubber component occurs when the concentration of Ca²⁺ is 0.1 mol/L ormore;(II) a method of preparing a deproteinized natural rubber latex, whichcomprises adding a protease and two or more surfactant shaving differentcoagulation properties to calcium ions (Ca²⁺) to a natural rubber latex,and maturing the natural rubber latex, wherein two or more surfactantsare stably dispersed when the concentration of Ca²⁺ of an aqueoussolution (25° C.) containing the surfactants is 0.1 mol/L or less, andare coagulated when Ca²⁺ of the aqueous solution is 1.0 mol/L or more;(III) a rubber product using a deproteinized natural rubber latex, whichis obtained by dipping a dipping mold, the surface of which is coatedwith an anode coagulant, in the deproteinized natural rubber latexcontaining a vulcanizing agent added therein described in the term (I),vulcanizing a rubber film formed on the surface of the dipping mold, andremoving the rubber film from the dipping mold; and(IV) a proteolytic agent for natural rubber latex, comprising a proteaseand two or more surfactants having different coagulation properties tocalcium ions (Ca²⁺), wherein

two or more surfactants are stably dispersed when the concentration ofCa²⁺ of an aqueous solution (25° C.) containing the surfactants is 0.1mol/L or less, and are coagulated when Ca²⁺ of the aqueous solution is1.0 mol/L or more.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes the respective inventions relating to thefollowing deproteinized natural rubber latex, the method of preparingthe same, the rubber product using the same, and the proteolytic agentfor deproteinized natural rubber latex.

(1) A deproteinized natural rubber latex which is obtained by subjectingto a treatment for decomposition and removal of a protein, whereincoagulation of a rubber component does not occur when the concentrationof calcium ions (Ca²⁺) is 0.01 mol/L or less and coagulation of therubber component occurs when the concentration of calcium ions (Ca²⁺) is0.1 mol/L or more.

(2) The deproteinized natural rubber latex described in the term (1),wherein the treatment for decomposition of the protein is conducted byadding a protease and two or more surfactants having differentcoagulation properties to calcium ions (Ca²⁺) to a natural rubber latexand maturing the natural rubber latex, and

two or more surfactants are stably dispersed when the concentration ofCa²⁺ of an aqueous solution (25° C.) containing the surfactants is 0.1mol/L or less, and are coagulated when Ca²⁺ of the aqueous solution is1.0 mol/L or more.

(3) The deproteinized natural rubber latex described in the term (1),wherein the deproteinized natural rubber latex, which is obtained bysubjecting to a treatment for decomposition and removal of a protein, isprepared by adding a protease to a natural rubber latex and maturing thenatural rubber latex, centrifuging the latex, thereby to isolate acreamy rubber solid content, and dispersing the rubber solid content inan aqueous medium,

wherein the aqueous medium contains two or more surfactants havingdifferent coagulation properties to calcium ions (Ca²⁺) and enables thesurfactants to stably disperse when a liquid temperature is 25° C. andthe concentration of calcium ions (Ca²⁺) is 0.1 mol/L or less, and tocoagulate when the liquid temperature is 25° C. and the concentration ofCa²⁺ is 1.0 mol/L or more.

(4) The deproteinized natural rubber latex described in the term (2) or(3), wherein two or more surfactants include:

at least one surfactant (sometimes referred to as “surfactant H”)selected from the group consisting of carboxylic acid anionicsurfactant, higher alcohol sulfate ester salt anionic surfactant,sulfonic acid anionic surfactant and phosphoric acid anionic surfactant,and

at least one surfactant (sometimes referred to as “surfactant L”)selected from the group consisting of higher alkylphenyl ether sulfateester salt anionic surfactant and higher alkyl ether sulfate ester saltanionic surfactant.

(5) The deproteinized natural rubber latex described in the term (2),wherein the total amount of two or more surfactants added on treatmentfor decomposition of a protein is within a range from 0.01 to 10 partsby weight based on 100 parts by weight of the rubber solid content ofthe natural rubber latex.

(6) The deproteinized natural rubber latex described in the term (3),wherein the total content of two or more surfactants in the aqueousdispersion medium is within a range from 0.01 to 10 parts by weightbased on 100 parts by weight of the solid content dispersed in theaqueous dispersion medium.

(7) A method of preparing a deproteinized natural rubber latex, whichcomprises adding a protease and two or more surfactants having differentcoagulation properties to calcium ions (Ca²⁺) to a natural rubber latex,and maturing the natural rubber latex, wherein two or more surfactantsare stably dispersed when the concentration of Ca²⁺ of an aqueoussolution (25° C.) containing the surfactants is 0.1 mol/L or less, andare coagulated when Ca²⁺ of the aqueous solution is 1.0 mol/L or more.

(8) A method of preparing a natural rubber latex, which comprisessubjecting a natural rubber latex to a treatment for decomposition of aprotein due to a protease and a treatment for removal of a protein dueto centrifugation, and dispersing the resulting creamy rubber solidcontent in an aqueous medium,

wherein the aqueous medium contains two or more surfactants havingdifferent coagulation properties to calcium ions (Ca²⁺), and enables thesurfactants to stably disperse when a liquid temperature is 25° C. andthe concentration of Ca²⁺ is 0.1 mol/L or less, and to coagulate whenthe liquid temperature is 25° C. and the concentration of Ca²⁺ is 1.0mol/L or more.

(9) A rubber product using a deproteinized natural rubber latex, whichis obtained by dipping a dipping mold, the surface of which is coatedwith an anode coagulant, in the deproteinized natural rubber latexcontaining a vulcanizing agent added therein described in any one ofterms (1) to (6), vulcanizing a rubber film formed on the surface of thedipping mold, and removing the rubber film from the dipping mold.

(10) A proteolytic agent for natural rubber latex, comprising a proteaseand two or more surfactant shaving different coagulation properties tocalcium ions (Ca²⁺), wherein

two or more surfactants are stably dispersed when the concentration ofCa²⁺ of an aqueous solution (25° C.) containing the surfactants is 0.1mol/L or less, and are coagulated when Ca²⁺ of the aqueous solution is1.0 mol/L or more.

(11) The proteolytic agent for natural rubber latex described in theterm (10), wherein two or more surfactants include:

at least one surfactant H selected from the group consisting ofcarboxylic acid anionic surfactant, higher alcohol sulfate ester saltanionic surfactant, sulfonic acid anionic surfactant and phosphoric acidanionic surfactant, and

at least one surfactant L selected from the group consisting of higheralkyl phenyl ether sulfate ester salt anionic surfactant and higheralkyl ether sulfate ester salt anionic surfactant.

(12) The proteolytic agent for natural rubber latex described in theterm (11), wherein a ratio of the content of the surfactant H to thesurfactant L is within a range from 15:85 to 70:30 by weight ratio.

According to the invention (1), it is made possible to produce a rubberproduct having a sufficient film thickness under the same conditions asthose in case of forming a film from a non-deproteinized natural rubberlatex by means of the anode coagulation method without causing uneventhickness of the rubber film and liquid dripping when a mold isrepeatedly dipped.

In case of the deproteinized natural rubber latex described in JapanesePublished Unexamined Patent (Kokai Tokkyo Koho) No. 2000-17002, therearises a problem that long-term stability of the latex is impaired byincorporating the both of the heat sensitizer and the anode coagulant inthe amount larger than that in case of a conventional formulation asdescribed above. However, according to the deproteinized natural rubberlatex of the present invention, such a problem is not likely to arise.

As used herein, the expression “coagulation of the rubber occurs” refersto the fact that the rubber component in the deproteinized naturalrubber latex is completed isolated in the upper layer of the latex inthe form of a coherent solid content (in such a case, an aqueoussolution containing Ca²⁺ becomes transparent) or the fact that thecoherent solid content is partially observed in the deproteinizednatural rubber latex (in such a case, the aqueous solution containing Ca²⁺ is still in the state of white turbidity).

As used herein, the expression “coagulation of the rubber does notoccur” refers to the fact that the rubber component in the deproteinizednatural rubber latex is maintained in the dispersed and suspended stateand the coherent rubber solid content is not observed in the latex (insuch a case, the aqueous solution containing Ca²⁺ is still in the stateof white turbidity).

The presence or absence of “coagulation of the rubber component” isjudged whether or not the rubber component in a deproteinized naturalrubber latex is observed in the form of a coherent solid content afteradding dropwise the latex in an aqueous solution wherein theconcentration of Ca²⁺ is controlled to a predetermined value. In case“coagulation of the rubber component does not occur”, the rubbercomponent is rapidly dispersed in the aqueous solution containing Ca²⁺when the latex is added dropwise.

Regarding the natural rubber latex used to evaluate the coagulationproperties to Ca²⁺, the concentration of the rubber solid content is notspecifically limited. However, since it becomes difficult to judgecoagulation of the rubber component when the concentration of the rubbersolid content is too low, the concentration of the solid rubber contentof the latex is preferably set within a range from about 30 to 60% byweight beforehand. The liquid temperature of the aqueous solution havinga predetermined concentration of Ca²⁺ used to evaluate the coagulationproperties to Ca²⁺ of the latex is not specifically limited, but ispreferably set to a fixed temperature of about 25° C. In the presentinvention, those prepared by controlling the concentration of the rubbersolid content to 60% by weight were used as the latex in case ofevaluating the coagulation properties to Ca²⁺ and the liquid temperatureof the aqueous solution having a predetermined concentration of Ca²⁺ wasset to 25° C., unless otherwise specified.

The deproteinized natural rubber latex of the invention (1) can beprepared by stabilizing the latex using a combination of two or moresurfactants having different coagulation properties to Ca²⁺. Thecoagulation properties to the Ca²⁺ of the deproteinized natural rubberlatex is set within a predetermined range by the combination of thesesurfactants.

More specifically, the deproteinized natural rubber latex of the presentinvention (1) can be prepared by the process (i) of subjecting a naturalrubber latex to a deproteinization treatment using two or moresurfactants having different coagulation properties to Ca²⁺ and aprotease, or the process (ii) of dispersing a natural rubber subjectedpreviously to a deproteinization treatment by various conventionallyknown methods in an aqueous dispersion medium wherein the coagulationproperties to Ca²⁺ are set within a predetermined range.

In the deproteinized natural rubber latex of the present invention, twoor more surfactants having different coagulation properties (dispersionstability) to Ca²⁺ are used for the purpose of maintaining the dispersedstate of the rubber component unstabilized as a result of thedeproteinization treatment.

In case the coagulation properties to the concentration of Ca²⁺ of thelatex are set within the above range using a combination of a surfactanthaving relatively high coagulation properties (low dispersibility) toCa²⁺ and a surfactant having relatively low coagulation properties (highdispersibility) to Ca²⁺, the stably-dispersed state of the rubbercomponent can be maintained without causing coagulation of the rubbercomponent in the latex at the stage before film formation by means ofthe anode coagulation method. Specific examples of the stage before filmformation by means of the anode coagulation method include storage ofthe deproteinized natural rubber latex for a long period of time,application of mechanical vibration to the latex during conveying, andaddition of various additives such as vulcanizing agent to the latex.

When the deproteinized natural rubber latex of the present invention isbrought into contact with an anode coagulant having the concentrationused conventionally in the anode coagulation method, coagulation occursin the surfactant having higher coagulation properties to Ca²⁺ betweentwo surfactants and the dispersibility of the rubber in the latex isdrastically lowered. Therefore, it is made possible to form a rubberfilm by means of the anode coagulation method.

As used herein, the expression “two or more surfactants are coagulated”refers to the fact that at least one of two surfactants is bonded withCa²⁺ to form a salt insoluble in water. As used herein, the expression“two or more surfactants are stably dispersed” refers to the fact thatthe dispersibility of the surfactants in the aqueous solution ismaintained without forming a salt as a result of bonding with Ca²⁺.

As used herein, the term “aqueous dispersion medium” mainly refers towater. As far as the dispersion stability of the latex are not adverselyaffected, the aqueous dispersion medium include those which containother solvents miscible with water (for example, organic solvent) andadditives incorporated conventionally in the latex used in filmformation by means of the anode coagulation method.

The coagulation properties to Ca²⁺ of the surfactant can be set withinthe above range by using those belonging to the surfactant H incombination with those belonging to the surfactant L as two or moresurfactants in the inventions (2) and (3).

Balance between the film forming properties by means of the anodecoagulation method and the dispersion stability with respect to thedeproteinized natural rubber latex can be further improved by settingthe total amount or total content of two or more surfactants to theabove range in the inventions (5) and (6).

According to the inventions (7) and (8), it is made possible to preparea deproteinized natural rubber latex wherein coagulation of a rubbercomponent does not occur when the concentration of calcium ions (Ca²⁺)is 0.01 mol/L or less and coagulation of the rubber component occurswhen the concentration of Ca²⁺ is 0.1 mol/L or more.

The method of preparing the deproteinized natural rubber latex of theinvention (7) is one aspect of the method of preparing the deproteinizednatural rubber latex according to the invention (2). The method ofpreparing the deproteinized natural rubber latex of the invention (8) isone aspect of the method of preparing the deproteinized natural rubberlatex according to the invention (3).

In the methods of preparing the deproteinized natural rubber latexaccording to the inventions (7) and (8), as two or more surfactants,those including at least one surfactant H selected from the groupconsisting of carboxylic acid anionic surfactant, higher alcohol sulfateester salt anionic surfactant, sulfonic acid anionic surfactant andphosphoric acid anionic surfactant, and at least one surfactant Lselected from the group consisting of higher alkyl phenyl ether sulfateester salt anionic surfactant and higher alkyl ether sulfate ester saltanionic surfactant are preferably used.

In the invention (7), the total amount of two or more surfactants addedon treatment for decomposition of a protein is preferably within a rangefrom 0.01 to 10 parts by weight based on 100 parts by weight of therubber solid content of the natural rubber latex. In the invention (8),the total content of two or more surfactants in the aqueous dispersionmedium is preferably within a range from 0.01 to 10 parts by weightbased on 100 parts by weight of the rubber solid content dispersed inthe aqueous dispersion medium.

In such a case, balance between the film forming properties by means ofthe anode coagulation method and the dispersion stability of a latex canbe further improved.

The rubber product of the invention (9) is produced by adding avulcanizing agent to the deproteinized natural rubber latex of thepresent invention, dipping a dipping mold, the surface of which iscoated with an anode coagulant, in the deproteinized natural rubberlatex, vulcanizing a rubber film formed on the surface of the dippingmold, and removing the rubber film from the dipping mold.

According to the rubber product using the deproteinized natural rubberlatex of the invention (9) and the method of producing the same, it ismade possible to produce a rubber product made of a deproteinizednatural rubber latex, wherein a fear of the occurrence of immediate (Itype) allergy has been markedly reduced by the deproteinizationtreatment, by using a non-deproteinized natural rubber latex under thesame conditions. Accordingly, the invention (9) is suited forapplication to a rubber glove for home use having a thickness of about 1mm.

As used herein, the term “rubber product” refers to a rubber productproduced by the anode coagulation method and specific examples thereofinclude medical appliances (for example, catheter, double balloon,etc.), finger cots and toys, including rubber gloves.

In the rubber product using the deproteinized natural rubber latex ofthe present invention and the method of producing the same, examples ofthe anode coagulant include, but are not limited to, metal salts havingan ionic value of 2 or more and organic alkylamine salts. Examples ofthe metal salt having an ionic value of 2 or more include calciumnitrate and calcium chloride. These anode coagulants are generally usedin the form of an aqueous solution.

Regarding the proteolytic agents for deproteinized natural rubber latexof the inventions (11) to (12), the degree of the coagulation to Ca²⁺ ofthe surfactant contained in the proteolytic agent is controlled withinthe above range, as described above. Therefore, in case the treatmentfor decomposition of a protein in the natural rubber latex is conductedusing the proteolytic agent of the present invention, proper anodecoagulation properties (in other words, excellent film formingproperties by means of the anode coagulation method) can be imparted tothe latex after subjecting to the treatment.

Regarding the proteolyzed natural rubber latex (or deproteinized naturalrubber latex) obtained by such a treatment, the stably-dispersed stateof the rubber component can be maintained without causing coagulation ofthe rubber component in the latex at the stage before film formation bymeans of the anode coagulation method. Specific examples of the stagebefore film formation by means of the anode coagulation method includestorage of the deproteinized natural rubber latex for a long period oftime, application of mechanical vibration to the latex during conveying,and addition of various additives such as vulcanizing agents to thelatex.

When the proteolyzed natural rubber latex (or deproteinized naturalrubber latex) of the present invention is brought into contact with ananode coagulant having the concentration used conventionally in theanode coagulation method, coagulation occurs in the surfactant havinghigher coagulation properties to Ca²⁺ among two surfactants and thedispersibility of the rubber in the latex is drastically lowered.Therefore, it is made possible to form a rubber film by means of theanode coagulation method.

Since the proteolytic agent of the present invention can control thesensitivity to anode coagulants such as calcium nitrate and calciumchloride of the proteolyzed natural rubber latex (or deproteinizednatural rubber latex) treated with the treating agent to propersensitivity, the proteolytic agent of the present invention is suited toprepare a proteolyzed natural rubber latex (or deproteinized naturalrubber latex) for film formation by means of the anode coagulationmethod.

In the proteolytic agent for natural rubber latex of the presentinvention, two or more surfactants are preferably those including atleast one surfactant H selected from the group consisting of carboxylicacid anionic surfactant, higher alcohol sulfate ester salt anionicsurfactant, sulfonic acid anionic surfactant and phosphoric acid anionicsurfactant, and at least one surfactant L selected from the groupconsisting of higher alkyl phenyl ether sulfate ester salt anionicsurfactant and higher alkyl ether sulfate ester salt anionic surfactantare preferably used.

The proteolytic agent for natural rubber latex of the present inventioncontains two or more surfactants having different coagulation properties(dispersion stability) to Ca²⁺, the degree of coagulation to Ca²⁺ beingcontrolled within a predetermined range. As described above, when usingthose having relatively high coagulation properties (low dispersibility)to Ca²⁺ in combination with those having relatively low coagulationproperties (high dispersibility) to Ca²⁺, the coagulation properties toCa²⁺ of the latex can be easily set within the range described above.Specific examples of the surfactant having relatively high coagulationproperties to Ca²⁺ include those included in the group of the surfactantH, while specific examples of the surfactant having relatively lowcoagulation properties to Ca²⁺ include those included in the group ofthe surfactant L.

The coagulation properties to Ca²⁺ with respect to two or moresurfactants are usually evaluated by adding dropwise the surfactant inthe form of an aqueous solution to an aqueous solution containing Ca²⁺.In this case, the concentration of the aqueous surfactant is notspecifically limited. However, since it becomes difficult to judge thepresence or absence of coagulation of the rubber component when theconcentration is too low, it is preferred to previously set theconcentration of the aqueous solution of two or more surfactants(mixture) to about 10% by weight. The liquid temperature of the aqueoussolution having a predetermined concentration of Ca²⁺ used to evaluatethe coagulation properties to Ca²⁺ of the latex is not specificallylimited, but is preferably set within a temperature range where the filmforming treatment is conducted by the anode coagulation method. Ingeneral, liquid temperature of the aqueous solution is preferably set toabout 25° C. in case of evaluating the degree of coagulation propertiesto the concentration of Ca²⁺. In the present invention, the liquidtemperature of the aqueous solution having a predetermined concentrationof Ca²⁺ was set to 25° C., unless otherwise specified.

In the proteolytic agent for natural rubber latex of the presentinvention, a ratio of the content of the surfactant H to the surfactantL is preferably within a range from 15:85 to 70:30 in a weight ratio.

The use of the surfactant H and the surfactant L in a weight ratiowithin a range from 15:85 to 70:30 makes it possible to easily controlthe degree of the coagulation properties to the Ca²⁺ of the surfactant.Accordingly, it is made possible to improve balance between the filmforming properties by means of the anode coagulation method and thestorage stability of the latex itself with respect to the proteolyzednatural rubber latex (or deproteinized natural rubber latex) treatedwith the proteolytic agent of the present invention.

Embodiments of the present invention will now be described.

[Deproteinized natural rubber latex and method of preparing the same]

(Natural Rubber Latex)

The natural rubber latex used to prepare the deproteinized naturalrubber latex of the present invention may be any of a field latexobtained as a rubber sap and an ammonia-retained concentrated latex.

(Protease)

In the present invention, the protease used in a treatment fordecomposition of a protein to the natural rubber latex is notspecifically limited and various conventionally known proteases can beused and, for example, an alkaline protease is preferable. The proteasemay be derived from any of bacteria, filamentous bacteria and yeast, andthe protease is preferably derived from bacteria, particularlypreferably from the genus Bacillus. It is also possible to use enzymessuch as lipase, esterase, amylase, lacase and cellulase in combination.

When using the alkaline protease, its activity [measured value obtainedby modification of the Anson-hemoglobin method (Anson. M. L. J. Gen.Physiol., 22, 79 (1938))] is within a range from 0.1 to 50 APU/g, andpreferably within a range from 1 to 25 APU/g.

The amount of the protease varies depending on the activity of theprotease itself, and is not specifically limited. In general, thecontent of the protease is preferably controlled within a range from0.0001 to 20 parts by weight, and more preferably within a range from0.001 to 10 parts by weight, based on 100 parts by weight of the rubbercomponent in the natural rubber latex. When the content of the proteaseis within the range described above, a protein in the latex can besufficiently decomposed while maintaining the activity of the protease.Alternatively, the effect corresponding to the amount of the proteasecan be exerted effectively and, therefore, it is advantageous in view ofthe cost.

(Surfactant)

The surfactant used to prepare the deproteinized natural rubber latex ofthe present invention is composed of a combination of two or moresurfactants having different coagulation properties to Ca²⁺. It isrequired for the combination of two or more surfactants to be set sothat the coagulation properties to Ca²⁺ with respect to the aqueoussolution containing two or more surfactants are within a predeterminedrange.

Specifically, it is required that, when a liquid temperature of anaqueous solution (or aqueous dispersion medium) containing two or moresurfactants is 25° C. and the concentration of Ca²⁺ is 0.1 mol/L orless, the surfactants are stably dispersed, whereas, when the liquidtemperature of the aqueous solution (or aqueous dispersion medium) is25° C. and the concentration of Ca²⁺ is 1.0 mol/L or more, thesurfactants are coagulated.

As the surfactant used in the present invention, for example,

at least one surfactant H selected from the group consisting ofcarboxylic acid anionic surfactant, higher alcohol sulfate ester saltanionic surfactant, sulfonic acid anionic surfactant and phosphoric acidanionic surfactant, and

at least one surfactant L selected from the group consisting of higheralkyl phenyl ether sulfate ester salt anionic surfactant and higheralkyl ether sulfate ester salt anionic surfactant may be used incombination.

Those included in the group of the surfactant H are surfactants havingrelatively high coagulation properties (relatively low dispersibility)to Ca²⁺, and those included in the group of the surfactant L aresurfactants having relatively low coagulation properties (relativelyhigh dispersibility) to Ca²⁺.

Specific examples of those included in the group of the surfactant H areshown in Table 1. Also, specific examples of those included in the groupof the surfactant L are shown in Table 2.

TABLE 1 *Surfactant H (having high coagulation properties to Ca²⁺) No.Kind and name of surfactants H-1 Carboxylic acid anionic surfactantH-1-1 Potassium oleate H-1-2 Sodium dialkylsuccinate H-1-3 Sodium oleateH-1-4 Sodium laurate H-1-5 Sodium stearate H-2 Higher alcohol sulfateester salt anionic surfactant H-2-1 Sodium laurate sulfate H-2-2 Sodiumcetyl sulfate H-2-3 sodium stearyl sulfate H-2-4 Sodium oleyl sulfateH-3 Sulfonic acid anionic surfactant H-3-1 Sodium dodecylbenzenesulfonate H-4 Phosphoric acid anionic surfactant H-4-1 Potassiumpolyoxyethylene nonylphenyl phosphate

TABLE 2 *Surfactant L (having low coagulation properties to Ca²⁺) No.Kind and name of surfactants L-1 Higher alkyl phenyl ether sulfate esteranionic surfactant L-1-1 Sodium POE nonyl phenyl ether sulfate L-2Higher alkyl ether sulfate ester salt anionic surfactant L-2-1 SodiumPOE alkyl ether sulfate POE: polyoxyethylene

Although a mixing ratio of the surfactant H to the surfactant L is notspecifically limited, a weight ratio (H:L) is preferably set within arange from 15:85 to 70:30.

When the total amount (total content) of the surfactant H and thesurfactant L is equal to 100, the lower limit of the addition amount(content) of the surfactant H is preferably 20 [H:L (weightratio)=20:80], and more preferably 25 [H:L (weight ratio)=25:75], withinthe range described above. On the other hand, the upper limit of thecontent of the surfactant H is preferably 65 [H:L (weight ratio) 65:35],and more preferably 60 [H:L (weight ratio) =60:40], within the rangedescribed above.

(Content of Protease and Surfactant)

In the present invention, especially second invention among the presentinvention, the content of the protease and that of the surfactant ontreatment for removal of a protein are not specifically limited. Toefficiently promote the treatment for decomposition of a protein, aratio of the both is preferably set within a range from 1:1 to 1:200,and more preferably from 1:10 to 1:50.

(Method of Treatment for Decomposition and Removal of Protein)

The treatment for decomposition of a protein to a natural rubber latexis conducted by adding a predetermined amount of the protease and apredetermined combination of the surfactants to a natural rubber as araw material and maturing the mixture for about several tens of minutesto one week, and preferably about 1 to 3 days.

This maturing treatment may be conducted while stirring the latex orallowing it to stand. If necessary, the temperature may be controlled.The temperature is preferably controlled within a range from 5 to 90°C., and more preferably from 20 to 60° C. to obtain sufficient activityof the enzyme. When the temperature is lower than 5° C., there is a fearthat the enzyme reaction does not proceed. On the other hand, when thetemperature exceeds 90° C., there is a fear that the enzyme isdevitalized.

The treatment for removal of a protein (or decomposition productthereof) after the decomposition of a protein includes, but is notspecifically limited to, a treatment of concentrating the latex bycentrifugation or ultrafiltration and separating the non-rubbercomponent transferred in water such as protein decomposition product andthe rubber particles in the latex, or a treatment of separating therubber particles by coagulation using an acid. Among these treatments,purification by centrifugation is most preferred in view of the accuracyand efficiency of purification.

The protease added to the natural rubber latex is washed and removed bythe above purification treatment after being subjected to the treatmentfor removal of a protein. Regarding the surfactants added to the naturalrubber latex, a portion of them is washed and removed by thepurification treatment. Although a portion of the surfactants isremained in the deproteinized natural rubber latex even after thepurification treatment and acts as a stabilizer of the latex, thestability of the deproteinized natural rubber latex is drasticallyimpaired when the residual amount is too small (when almost all of themis removed by the purification treatment). However, when the treatmentof washing (purification) of the latex after the deproteinizationtreatment is conducted by a conventional centrifugation method underconventional treatment conditions, that is, a washing (purification)treatment is conducted under the conditions where the protease and thedecomposition product of the protein can be washed and removed, it isnot necessary to add a new surfactant to the latex after thedeproteinization treatment.

More specifically, in case the cleaning (purification) treatment isconducted by a centrifugation treatment, sufficient stability andheat-sensitive coagulation properties of the latex can be maintained bydispersing a cream component separated in the upper layer bycentrifugation under the conditions of 5,000 to 14,000 rpm for about 1to 60 minutes again in water having almost the same volume as that ofthe cream component even after the deproteinization treatment using thewater-soluble polymer which is previously added before thedeproteinization treatment.

In case of preparing a deproteinized natural rubber latex of the thirdinvention among the present invention, it is necessary to sufficientlytake notice of the amount of the surfactant used in the treatment fordecomposition of a protein to the natural rubber latex. The reason is asfollows. That is, when the surfactant remains in a coagulated rubberobtained by the deproteinization treatment, it is likely to exert anadverse influence on the anode coagulation properties (sensitivity toanode coagulant) of the deproteinized natural rubber latex. In case ofthe deproteinized natural rubber latex of the third invention, since thelatex is prepared by dispersing the solid rubber component obtained bythe deproteinization treatment in the aqueous solution containing apredetermined surfactant, any problem does not arise even if the rubbercomponent is coagulated on deproteinization treatment. Accordingly, ifthe protein and decomposition product thereof can be sufficientlyremoved, the surfactant may not be added on deproteinization treatment.

(Degree of Deproteinization)

Although the degree of the deproteinization in the deproteinized naturalrubber latex used in the present invention is not specifically limited,it is required to control so that the nitrogen content (N %) asdetermined by the Kjeldahl method after the deproteinization treatmentis 0.1% or less, preferably 0.05% or less, and more preferably 0.02% orless, in order to make a final rubber product low-allergic. When thenitrogen content exceeds the above range, there is a fear that theoccurrence of the allergic reaction cannot be sufficiently suppressed inuse of a final rubber product because of insufficient degree of thedeproteinization.

The degree of the deproteinization can also be confirmed by the presenceor absence of adsorption and degree of adsorption on the basis of theprotein by means of an infrared absorption spectrum. In the rubbertreated by using the proteolytic agent of the present invention, anabsorption at 3320 cm⁻¹ derived from short-chain peptide or amino acidmay be observed. However, it is preferable that an absorption at 3280cm⁻¹ derived from polymer peptide as a cause for allergy is small. It ismore preferable that any absorption at 3280 cm⁻¹ is not observed.

[Rubber Product Using Deproteinized Natural Rubber Latex]

The rubber product using the deproteinized natural rubber latex of thepresent invention is obtained by dipping a dipping mold, the surface ofwhich is coated with an anode coagulant previously, in the deproteinizednatural rubber latex of the present invention to form a rubber filmformed on the surface of the dipping mold, vulcanizing the rubber film,and removing the rubber film from the dipping mold.

Examples of the anode coagulant, with which the surface of the dippingmold is previously coated, include, but are not limited to, metal saltshaving an ionic value of 2 or more and organic alkylamine salts.Examples of the metal salt having an ionic value of 2 or more includecalcium nitrate and calcium chloride. These anode coagulants aregenerally used in the form of an aqueous solution.

The concentration of the anode coagulant on film formation maybe setaccording to a conventional method and is not specifically limited, butis usually set within a range from 5 to 20% by weight, and preferablyfrom 10 to 15% by weight. This concentration is within a range fromabout 0.3 to 1.2 mol/L, and preferably from about 0.6 to 0.9 mol/L, whenreduced based on the concentration of Ca²⁺ assuming that the anodecoagulant is calcium nitrate (molecular weight: 164).

The dipping mold used in the production of the rubber product variesdepending on the shape of the objective rubber product. For example,when the rubber product is a rubber glove, a conventionally known handmold may be used as the dipping mold.

The film forming conditions may be set by a conventional methodaccording to the kind of the objective rubber product and the thicknessof the rubber film.

Vulcanizing agents are previously added to the deproteinized naturalrubber latex used in the production of the rubber products describedabove. If necessary, various additives, for example, vulcanizationchemicals such as vulcanization accelerators, auxiliary vulcanizationaccelerators and vulcanization retardants, and various additives such asfillers can be incorporated.

Examples of the vulcanizing agent include sulfur, and organicsulfur-containing compounds such as trimethylthiourea andN,N′-diethylthiourea, and these vulcanizing agents can be used alone orin combination. The amount of the vulcanizing agent is decided by evenbalance between the vulcanization degree and the amount of thevulcanization accelerator, but is usually set within a range from 0.1 to5 parts by weight, and preferably from 0.5 to 2 parts by weight, basedon 100 parts by weight of the rubber solid content in the rubber latex.

Examples of the vulcanization accelerator include zincN-ethyl-N-phenyldithiocarbamate (PX), zinc dimethyldithiocarbamate (PZ),zinc diethyldithiocarbamate (EZ), zinc dibutyldithiocarbamate (BZ), zincsalt of 2-mercaptobenzothiazole (MZ) and tetramethylthiuram disulfide(TT). These vulcanization accelerators can be used alone or incombination. The amount of the vulcanization accelerator is preferablyset within a range from about 0.5 to 3 parts by weight based on 100parts by weight of the rubber solid content in the rubber latex.Examples of the auxiliary vulcanization accelerator include zinc white.The amount of the auxiliary vulcanization accelerator is preferably setwithin a range from about 0.5 to 3 parts by weight based on 100 parts byweight of the rubber solid content in the rubber latex.

Examples of the filler include kaolin clay, hard clay and calciumcarbonate. The amount of the filler is preferably 10 parts by weight orless based on 100 parts by weight of the rubber solid content in therubber latex.

It is not necessary to incorporate a specific heat sensitizer and aspecific anode coagulant into the deproteinized natural rubber latex ina larger amount as compared with a conventional formulation, like thedeproteinized natural rubber latex described Japanese PublishedUnexamined Patent (Kokai Tokkyo Koho) No. 2000-17002. Therefore,according to the rubber product using the deproteinized natural rubberlatex of the present invention, it is made possible to prepare ahigh-quality rubber product, which is less contaminated with impurities,while reducing the material cost.

EXAMPLES

The following Examples and Comparative Examples further illustrate thepresent invention.

Example 1

(1) Preparation of Deproteinized Natural Rubber Latex

A high ammonia latex of a natural rubber was diluted so that theconcentration of the rubber component becomes 30% by weight. Adeproteinizing agent comprising a protease and a surfactant was added inthe amount of 1% by weight based on the rubber content of the latex.Then, the mixture was matured by standing while maintaining the liquidtemperature at 30° C. for 24 hours, thereby subjecting to a treatmentfor decomposition of a protein.

As the deproteinizing agent, a mixture of 2 parts by weight of aprotease [alkali protease, manufactured by Novo-Nordisk Bioindustri A/Sunder the trade name of “Alcalase 2.0M”], 49 parts by weight ofpotassium oleate [surfactant H (No. H-1-1) shown in Table 1] and 49parts by weight of sodium polyoxyethylene nonyl phenyl ether sulfate[surfactant L (No. L-1-1) shown in Table 2] was used.

After the completion of the treatment for decomposition of a protein,the latex was subjected to a centrifugation treatment at 13,000 rpm for30 minutes and the cream component separated in the upper layer wasdispersed again in water having the same volume as that of the creamcomponent to obtain a deproteinized natural rubber latex.

(2) Formation of Rubber Film (Production of Rubber Glove)

Based on 100 parts by weight of the rubber solid content in thedeproteinized natural rubber latex, 1 part by weight of colloidal sulfurdispersed in water, 0.5 parts by weight of zinc white and 1 part byweight of a vulcanization accelerator (zinc dibutyldithiocarbamate (BZ),manufactured by OUCHISHINKO CHEMICAL INDUSTRIAL CO., LTD. under thetrade name of “NOCCELER Bz”) were added, followed by maturing(pre-vulcanization) at 40° C. for 24 hours.

After previously coating the surface of a glass dipping mold (hand mold)with an aqueous 15 wt % solution of calcium nitrate (anode coagulant),this mold was dipped in the vulcanized latex for 10 seconds to form arubber film on the surface of the mold.

After forming the rubber film, the mold drawn up from the vulcanizedlatex was allowed to stand at room temperature (about 25° C.) for 60seconds and dipped again (double dipping) in the vulcanized latex for 10seconds.

Furthermore, the rubber film formed on the surface of the mold wasvulcanized by heating to 100° C. and the rubber film was removed fromthe dipping mold to obtain a rubber product (rubber glove).

Example 2

In the same manner as in Example 1, except that a mixture of 2 parts byweight of an alkali protease, 24 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 74 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the deproteinizing agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Example 3

In the same manner as in Example 1, except that sodium lauryl sulfate[surfactant H (No. H-2-1) shown in Table 1] was used as the surfactant Hin place of potassium oleate, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

Example 4

In the same manner as in Example 1, except that a mixture of 2 parts byweight of an alkali protease, 24 parts by weight of sodium laurylsulfate [surfactant H (No. H-2-1) and 74 parts by weight of sodiumpolyoxyethylene alkyl ether sulfate [surfactant L (No. L-1-2) shown inTable 2] was used as the deproteinizing agent, “(1) Preparation ofdeproteinized natural rubber latex” and “(2) Formation of rubber film”were conducted.

Example 5

In the same manner as in Example 1, except that 2 parts by weight of analkali protease, 49 parts by weight of sodium lauryl sulfate [surfactantH (No. H-2-1) and sodium polyoxyethylene alkyl ether sulfate [surfactantL (No. L-1-2)] were used as the deproteinizing agent, “(1) Preparationof deproteinized natural rubber latex” and “(2) Formation of rubberfilm” were conducted.

Comparative Example 1

In the same manner as in Example 1, except that 2 parts by weight of analkali protease and 98 parts by weight of potassium oleate [surfactant H(No. H-1-1) were used as the deproteinizing agent (that is, thesurfactant H was used alone), “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 2

In the same manner as in Example 1, except that 2 parts by weight of analkali protease and 98 parts by weight of sodium polyoxyethylene nonylphenyl ether sulfate [surfactant L (No. L-1-1) were used as thedeproteinizing agent (that is, the surfactant L was used alone), “(1)Preparation of deproteinized natural rubber latex” and “(2) Formation ofrubber film” were conducted.

Comparative Example 3

In the same manner as in Example 1, except that a mixture of 2 parts byweight of an alkali protease, 74 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 24 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the deproteinizing agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 4

In the same manner as in Example 1, except that a mixture of 2 parts byweight of an alkali protease, 12 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 86 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

(Evaluation of Coagulation Properties to Ca²⁺)

(i) With respect to the surfactant used in the treatment fordecomposition of a protein in Examples 1 to 5 and Comparative Examples 1to 4, the coagulation properties to Ca²⁺ were evaluated. Evaluation wasconducted in the following procedure. That is, the surfactant used inthe treatment for decomposition of a protein was dissolved in water inthe same content as that in the deproteinizing agent, thereby to controlthe concentration to 10% by weight, and then an aqueous solution of thesurfactant was added dropwise in an aqueous solution (25° C.) containingCa²⁺. The concentration of Ca²⁺ measured includes 0.1 mol/L and 1.0mol/L. The evaluation results are as shown in Table 3.

Each content of the surfactant shown in Table 3 is an approximate valuewhich shows the content of the surfactant in the deproteinizing agentshown in Table 4.

(ii) With respect to the deproteinized natural rubber latexes obtainedin Examples 1 to 5 and Comparative Examples 1 to 4, the coagulationproperties to Ca²⁺ were evaluated. Evaluation was conducted in thefollowing procedure. That is, the concentration of the rubber solidcontent in the deproteinized natural rubber latex was controlled to 60%by weight, and then the latex was added dropwise in an aqueous solution(25° C.) containing Ca²⁺. The concentration of Ca²⁺ measured includes0.01 mol/L and 0.1 mol/L. The evaluation results are as shown in Table4.

(Measurement of Nitrogen Content)

With respect to the deproteinized natural rubber latexes obtained inExamples 1 to 5 and Comparative Examples 1 to 4, the nitrogen content (N%) was measured by the Kjeldahl method. The measurement results are asshown in the column of “N %” in Table 4.

(Evaluation of Physical Properties of Vulcanized Rubber Film)

With respect to the rubber products (rubber products) obtained in theExamples 1 to 5 and Comparative Examples 1 to 4, the thickness anduniformity of the rubber film were evaluated. In accordance with thetest procedure defined in JIS K 6301, the tensile strength T_(B) (MPa)and the elongation at break E_(B) (%) were determined. The measurementresults are as shown in Table 5.

TABLE 3 *Surfactant used in treatment for decomposition of proteinCoagulation Content properties to Ca²⁺ of surfactant 0.1 mol/L 1.0 mol/LExample 1 H-1-1   50% dispersed coagulated L-1-1   50% Example 2 H-1-124.5% dispersed coagulated L-1-1 75.5% Example 3 H-2-1   50% dispersedcoagulated L-1-1   50% Example 4 H-2-1 24.5% dispersed coagulated L-1-275.5% Example 5 H-2-1   50% dispersed coagulated L-1-2   50% Comp.Example 1 H-1-1  100% coagulated coagulated Comp. Example 2 L-1-1  100%dispersed dispersed Comp. Example 3 H-1-1 75.5% coagulated coagulatedL-1-1 24.5% Comp. Example 4 H-1-1 12.2% dispersed dispersed L-1-1 87.8%(Note) Both symbols “H-” and “L-” in the column of “Content ofsurfactant” denote surfactants described in Table 1 and Table 2.

TABLE 4 Content of each component of Characteristics of deproteinizednatural deproteinizing rubber latex agent (weight Coagulation propertiesto Ca²⁺ ratio) N% 0.01 mol/L 0.1 mol/L Example 1 Protease 2 0.019non-coagulated coagulated H-1-1 49 L-1-1 49 Example 2 Protease 2 0.021non-coagulated coagulated H-1-1 24 L-1-1 74 Example 3 Protease 2 0.018non-coagulated coagulated H-2-1 49 L-1-1 49 Example 4 Protease 2 0.019non-coagulated coagulated H-2-1 24 L-1-2 74 Example 5 Protease 2 0.020non-coagulated coagulated H-2-1 49 L-1-2 49 Comp. Protease 2 0.019coagulated coagulated Example 1 H-1-1 98 Comp. Protease 2 0.020non-coagulated non-coagulated Example 2 L-1-1 98 Comp. Protease 2 0.020coagulated coagulated Example 3 H-1-1 74 L-1-1 24 Comp. Protease 2 0.019non-coagulated non-coagulated Example 4 H-1-1 12 L-1-1 86 (Note) Bothsymbols “H-” and “L-” in the column of “Content of each component ofdeproteinizing agent” denote surfactants described in Table 1 and Table2.

TABLE 5 Properties of vulcanized rubber film Film Tensile Elongationthickness Film strength at break (mm) uniformity T_(B) (MPa) E_(B) (%)Example 1 0.38 good 27.4 960 Example 2 0.37 good 27.1 950 Example 3 0.37good 26.8 950 Example 4 0.37 good 27.7 970 Example 5 0.38 good 26.8 950Comp. 0.40 poor 27.8 980 Example 1 Comp. 0.32 poor 27.1 960 Example 2Comp. 0.39 poor 27.1 940 Example 3 Comp. 0.33 poor 27.3 950 Example 4

As is apparent from Table 3 to Table 5, regarding all deproteinizednatural rubber latexes obtained in Examples 1 to 5, that is, thedeproteinized natural rubber latexes obtained by subjecting to thetreatment for decomposition of a protein using the surfactant H havingrelatively high coagulation properties to Ca²⁺ in combination with thesurfactant L having relatively low coagulation properties to Ca²⁺, arubber component was not coagulated when the concentration of calciumions (Ca²⁺) is 0.01 mol/L or less and the rubber component wascoagulated when the concentration of Ca²⁺ is 0.1 mol/L or more.

Therefore, it has been found that the deproteinized natural rubberlatexes obtained in Examples 1 to 5 are suited to form a film by theanode coagulation method and are capable of conducting film formation ofa rubber product having a sufficient film thickness by the anodecoagulation method, as is apparent from the results of “film thickness”and “film uniformity”.

To the contrary, in case Comparative Example 1 wherein only thesurfactant H was used and Comparative Example 3 wherein the content ofthe surfactant H was extremely large, since the rubber component wascoagulated when the concentration of Ca²⁺ is 0.01 mol/L or less,sufficient film formation could not conducted, thereby causing a problemsuch as poor uniformity of the film.

In case Comparative Example 2 wherein only the surfactant L was used andComparative Example 4 wherein the content of the surfactant L wasextremely larger than that of the surfactant H, since the rubbercomponent was not coagulated even when the concentration of Ca²⁺ is 0.1mol/L or more, sufficient film formation could not conducted, therebycausing such a problem that a film having a sufficient film thicknesscan not be uniformly formed.

Example 6

(1) Preparation of Deproteinized Natural Rubber Latex

A high ammonia latex of a natural rubber was diluted so that theconcentration of the rubber component becomes 30% by weight. Aproteolytic agent comprising a protease and a surfactant was added inthe amount of 1% by weight based on the rubber content of the latex.Then, the mixture was matured by standing while maintaining the liquidtemperature at 30° C. for 24 hours, thereby subjecting to a treatmentfor decomposition of a protein.

As the proteolytic agent, a mixture of 2 parts by weight of a protease[alkali protease, manufactured by Novo-Nordisk Bioindustri A/S under thetrade name of “Alcalase 2.0M”], 49 parts by weight of potassium oleate[surfactant H (No. H-1-1) shown in Table 1] and 49 parts by weight ofsodium polyoxyethylene nonyl phenyl ether sulfate [surfactant L (No.L-1-1) shown in Table 2] was used.

After the completion of the treatment for decomposition of a protein,the latex was subjected to a centrifugation treatment at 13,000 rpm for30 minutes and the cream component separated in the upper layer wasdispersed again in water having the same volume as that of the creamcomponent to obtain a deproteinized natural rubber latex.

(2) Formation of Rubber Film

Based on 100 parts by weight of the rubber solid content in thedeproteinized natural rubber latex, 1 part by weight of colloidal sulfurdispersed in water, 0.5 parts by weight of zinc white and 1 part byweight of a vulcanization accelerator (zinc dibutyldithiocarbamate (BZ),manufactured by OUCHISHINKO CHEMICAL INDUSTRIAL CO., LTD. under thetrade name of “NOCCELER Bz”) were added, followed by maturing(pre-vulcanization) at 40° C. for 24 hours.

After previously coating the surface of a glass dipping mold (hand mold)with an aqueous 15 wt % solution of calcium nitrate (anode coagulant),this mold was dipped in the vulcanized latex for 10 seconds to form arubber film on the surface of the mold.

After forming the rubber film, the mold drawn up from the vulcanizedlatex was allowed to stand at room temperature (about 25° C.) for 60seconds and dipped again (double dipping) in the vulcanized latex for 10seconds.

Furthermore, the rubber film formed on the surface of the mold wasvulcanized by heating to 100° C. and the rubber film was removed fromthe dipping mold to obtain a rubber product.

Example 7

In the same manner as in Example 6, except that a mixture of 2 parts byweight of an alkali protease, 24 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 74 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Example 8

In the same manner as in Example 6, except that sodium lauryl sulfate[surfactant H (No. H-2-1) shown in Table 1] was used as the surfactant Hin place of potassium oleate, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

Example 9

(1) Preparation of Deproteinized Natural Rubber Latex

The same amount of the proteolytic agent as that used in Example 6 wasadded to a high ammonia latex of a natural rubber diluted so that theconcentration of the rubber component becomes 30% by weight and thetreatment for decomposition of a protein was conducted under the sameconditions.

After the completion of the treatment, the resulting proteolyzed naturalrubber latex was used as it is without subjecting to the treatment forremoval of a decomposition product due to a centrifugation treatment.

(2) Formation of Rubber Film and Evaluation of Film Forming Propertiesand Physical Properties

In the same manner as in Example 6, except that the proteolyzed naturalrubber latex was used in place of the deproteinized natural rubberlatex, “(2) Formation of rubber film” was conducted.

Example 10

In the same manner as in Example 9, except that a mixture (which is thesame proteolytic agent as that of Example 7) of 2 parts by weight of analkali protease, 24 parts by weight of potassium oleate [surfactant H(No. H-1-1) and 74 parts by weight of sodium polyoxyethylene nonylphenyl ether sulfate [surfactant L (No. L-1-1)] was used as theproteolytic agent, “(1) Preparation of deproteinized natural rubberlatex” and “(2) Formation of rubber film” were conducted.

Example 11

In the same manner as in Example 9, except that a mixture (which is thesame proteolytic agent as that of Example 8) of 2 parts by weight of analkali protease, 24 parts by weight of sodium lauryl sulfate [surfactantH (No. H-2-1) and 74 parts by weight of sodium polyoxyethylene nonylphenyl ether sulfate [surfactant L (No. L-1-1)] was used as theproteolytic agent, “(1) Preparation of deproteinized natural rubberlatex” and “(2) Formation of rubber film” were conducted.

Comparative Example 5

In the same manner as in Example 6, except that 2 parts by weight of analkali protease and 98 parts by weight of potassium oleate [surfactant H(No. H-1-1) were used as the proteolytic agent (that is, the surfactantH was used alone), “(1) Preparation of deproteinized natural rubberlatex” and “(2) Formation of rubber film” were conducted.

Comparative Example 6

In the same manner as in Example 6, except that 2 parts by weight of analkali protease and 98 parts by weight of sodium polyoxyethylene nonylphenyl ether sulfate [surfactant L (No. L-1-1) were used as theproteolytic agent (that is, the surfactant L was used alone), “(1)Preparation of deproteinized natural rubber latex” and “(2) Formation ofrubber film” were conducted.

Comparative Example 7

In the same manner as in Example 6, except that a mixture of 2 parts byweight of an alkali protease, 74 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 24 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 8

In the same manner as in Example 6, except that a mixture of 2 parts byweight of an alkali protease, 12 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 86 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 9

In the same manner as in Example 9, except that a mixture (which is thesame proteolytic agent as that of Comparative Example 5) of 2 parts byweight of an alkali protease and 98 parts by weight of potassium oleate[surfactant H (No. H-1-1) was used as the proteolytic agent, “(1)Preparation of deproteinized natural rubber latex” and “(2) Formation ofrubber film” were conducted.

Comparative Example 10

In the same manner as in Example 9, except that a mixture (which is thesame proteolytic agent as that of Comparative Example 6) of 2 parts byweight of an alkali protease and 98 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant H (No. L-1-1) wasused as the proteolytic agent, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 11

In the same manner as in Example 9, except that a mixture (which is thesame proteolytic agent as that of Comparative Example 7) of 2 parts byweight of an alkali protease, 74 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 24 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant H (No. L-1-1) wasused as the proteolytic agent, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 12

In the same manner as in Example 9, except that a mixture (which is thesame proteolytic agent as that of Comparative Example 8) of 2 parts byweight of an alkali protease, 12 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 86 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant H (No. L-1-1) wasused as the proteolytic agent, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

(Evaluation of Coagulation Properties to Ca²⁺)

(i) With respect to the surfactant used in the treatment fordecomposition of a protein in Examples 6 to 11 and Comparative Examples5 to 12, the coagulation properties to Ca²⁺ with respect to thesurfactant contained in the proteolytic agent were evaluated. Evaluationwas conducted in the following procedure. That is, the surfactant usedin the treatment for decomposition of a protein was dissolved in waterin the same content as that in the proteolytic agent, thereby to controlthe concentration to 10% by weight, and then an aqueous solution of thesurfactant was added dropwise in an aqueous solution (25° C.) containingCa²⁺. The concentration of Ca²⁺ measured includes 0.1 mol/L and 1.0mol/L. The evaluation results are as shown in Table 6.

Each content of the surfactant shown in Table 6 is an approximate valuewhich shows the content of the surfactant in the proteolytic agent shownin Table 7 and Table 8.

(ii) With respect to the deproteinized natural rubber latexes obtainedin Examples 6 to 11 and Comparative Examples 5 to 12, the coagulationproperties to Ca²⁺ were evaluated. Evaluation was conducted in thefollowing procedure. That is, the concentration of the rubber solidcontent in the deproteinized natural rubber latex was controlled to 60%by weight, and then the latex was added dropwise in an aqueous solution(25° C.) containing Ca²⁺. The concentration of Ca²⁺ measured includes0.01 mol/L and 0.1 mol/L. The evaluation results are as shown in Table 7and Table 8.

From a viewpoint of achievement of good balance between the film formingproperties by means of the anode coagulation method and the dispersionstability of the latex itself, the coagulation properties to Ca²⁺ withrespect to the deproteinized natural rubber latex or proteolyzed naturalrubber latex are preferably those wherein coagulation of a rubbercomponent does not occur when the concentration of Ca²⁺ is 0.01 mol/L orless and coagulation of the rubber component occurs when theconcentration of Ca²⁺ is 0.1 mol/L or more.

(Measurement of Nitrogen Content)

With respect to the deproteinized natural rubber latexes obtained inExamples 6 to 8 and Comparative Examples 5 to 8, the nitrogen content (N%) was measured by the Kjeldahl method. The measurement results are asshown in the column of “N %” in Table 7.

(Evaluation of Physical Properties of Vulcanized Rubber Film)

With respect to the rubber products obtained in the Examples 6 to 11 andComparative Examples 5 to 12, the thickness and uniformity of the rubberfilm were evaluated. In accordance with the test procedure defined inJIS K 6301, the tensile strength T_(B) (MPa) and the elongation at breakE_(B) (%) were determined. The measurement results are as shown in Table9.

TABLE 6 *Surfactant used in treatment for decomposition of proteinCoagulation Content of properties to Ca2⁺ surfactant 0.1 mol/L 1.0 mol/LExample 6 & 9 H-1-1   50% dispersed coagulated L-1-1   50% Example 7 &10 H-1-1 24.5% dispersed coagulated L-1-1 75.5% Example 8 & 11 H-2-1  50% dispersed coagulated L-1-1   50% Comp. Example 5 & 9 H-1-1  100%coagulated coagulated Comp. Example 2 & 10 L-1-1  100% disperseddispersed Comp. Example 7 & 11 H-1-1 75.5% coagulated coagulated L-1-124.5% Comp. Example 8 & 12 H-1-1 12.2% dispersed dispersed L-1-1 87.8%(Note) Both symbols “H-” and “L-” in the column of “Content ofsurfactant” denote surfactants described in Table 1 and Table 2.

TABLE 7 Content of each component of Characteristics of deproteinizednatural proteolytic rubber latex agent Coagulation properties to Ca²⁺(weight ratio) N% 0.01 mol/L 0.1 mol/L Example 6 Protease 2 0.019non-coagulated coagulated H-1-1 49 L-1-1 49 Example 7 Protease 2 0.021non-coagulated coagulated H-1-1 24 L-1-1 74 Example 8 Protease 2 0.018non-coagulated coagulated H-2-1 49 L-1-1 49 Comp. Protease 2 0.019coagulated coagulated Example 5 H-1-1 98 Comp. Protease 2 0.020non-coagulated non-coagulated Example 6 L-1-1 98 Comp. Protease 2 0.020coagulated coagulated Example 7 H-1-1 74 L-1-1 24 Comp. Protease 2 0.019non-coagulated non-coagulated Example 8 H-1-1 12 L-1-1 86 (Note) Bothsymbols “H-” and “L-” in the column of “Content of each component ofdeproteinizing agent” denote surfactants described in Table 1 and Table2.

TABLE 8 Content of each component of Characteristics of deproteinizedproteolytic natural rubber latex agent Coagulation properties to Ca²⁺(weight ratio) N% 0.01 mol/L 0.1 mol/L Example 9 Protease 2 —non-coagulated coagulated H-1-1 49 L-1-1 49 Example Protease 2 —non-coagulated coagulated 10 H-1-1 24 L-1-1 74 Example Protease 2 —non-coagulated coagulated 11 H-2-1 49 L-1-1 49 Comp. Protease 2 —coagulated coagulated Example 9 H-1-1 98 Comp. Protease 2 —non-coagulated non-coagulated Example L-1-1 98 10 Comp. Protease 2 —coagulated coagulated Example H-1-1 74 11 L-1-1 24 Comp. Protease 2 —non-coagulated non-coagulated Example H-1-1 12 12 L-1-1 86 (Note) Bothsymbols “H-” and “L-” in the column of “Content of each component ofdeproteinizing agent” denote surfactants described in Table 1 and Table2.

TABLE 9 Film Tensile Elongation thickness Film strength at break (mm)uniformity T_(B) (MPa) E_(B) (%) Example 6 0.38 good 27.4 960 Example 70.37 good 27.1 950 Example 8 0.37 good 26.8 950 Comp. 0.40 poor 27.8 980Example 5 Comp. 0.32 poor 27.1 960 Example 6 Comp. 0.39 poor 27.1 940Example 7 Comp. 0.33 poor 27.3 950 Example 8 Example 9 0.38 good 26.5920 Example 10 0.37 good 25.7 930 Example 11 0.37 good 25.8 930 Comp.0.38 poor 26.0 910 Example 9 Comp. 0.36 poor 24.1 960 Example 10 Comp.0.39 poor 25.1 940 Example 11 Comp. 0.31 poor 24.3 920 Example 12

As is apparent from Table 6 to Table 9, regarding all proteolyzednatural rubber latexes or deproteinized natural rubber latexes obtainedby using the proteolytic agents of Examples 6 to 11 (using thesurfactant H having relatively high coagulation properties to Ca²⁺ incombination with the surfactant L having relatively low coagulationproperties) to Ca²⁺, coagulation did not occur when the concentration ofCa²⁺ is 0.01 mol/L or less and coagulation occurred when theconcentration of Ca²⁺ is 0.1 mol/L or more.

Therefore, it has been found that the proteolytic agents of Examples 6to 11 are suited to form a film by the anode coagulation method and arecapable of conducting film formation of a rubber product having asufficient film thickness by the anode coagulation method, as isapparent from the results of “film thickness” and “film uniformity”.

To the contrary, in case Comparative Examples 1 and 5 wherein only thesurfactant H was used and Comparative Examples 7 and 11 wherein thecontent of the surfactant H was extremely larger than that of thesurfactant L, since the coagulation occurred when the concentration ofCa²⁺ is 0.01 mol/L, sufficient film formation could not conducted,thereby causing a problem such as poor uniformity of the film.

In case Comparative Examples 6 and 10 wherein only the surfactant L wasused and Comparative Examples 8 and 12 wherein the content of thesurfactant L was extremely larger than that of the surfactant H, sincecoagulation did not occurred even when the concentration of Ca²⁺ is 0.1mol/L, sufficient film formation could not conducted, thereby causingsuch a problem that a film having a sufficient film thickness can not beuniformly formed.

1. A deproteinized natural rubber latex which is obtained by subjectinga natural rubber latex to a treatment for decomposition and removal of aprotein, wherein the treatment for decomposition of a protein isconducted by adding a protease and two or more anionic surfactantshaving different coagulation properties to calcium ions (Ca²⁺) to anatural rubber latex and maturing the natural rubber latex, and the twoor more surfactants are stably dispersed when the concentration of Ca²⁺in an aqueous solution at 25° C. containing the surfactants is 0.1 mol/Lor less, and are coagulated when the concentration of Ca²⁺ in theaqueous solution is 1.0 mol/L or more.
 2. The deproteinized naturalrubber latex according to claim 1, wherein the deproteinized naturalrubber latex, which is obtained by subjecting to a treatment fordecomposition and removal of a protein, is prepared by adding a proteaseto a natural rubber latex and maturing the natural rubber latex,centrifuging the latex, thereby to isolate a creamy rubber solidcontent, and dispersing the rubber solid content in an aqueous medium,wherein the aqueous medium contains two or more anionic surfactantshaving different coagulation properties to calcium ions (Ca²⁺), andenables the surfactants to stably disperse when a liquid temperature is25° C. and the concentration of Ca²⁺ is 0.1 mol/L or less, and tocoagulate when the liquid temperature is 25° C. and the concentration ofCa²⁺ is 1.0 mol/L or more.
 3. The deproteinized natural rubber latexaccording to claim 1 or 2, wherein the two or more surfactants include:at least one surfactant H selected from the group consisting ofcarboxylic acid anionic surfactant, higher alcohol sulfate ester saltanionic surfactant, sulfonic acid anionic surfactant and phosphoric acidanionic surfactant, and at least one surfactant L selected from thegroup consisting of higher alkyl phenyl ether sulfate ester salt anionicsurfactant and higher alkyl ether sulfate ester salt anionic surfactant.4. The deproteinized natural rubber latex according to claim 1, whereinthe total amount of two or more surfactants added on treatment fordecomposition of a protein is within a range from 0.01 to 10 parts byweight based on 100 parts by weight of the rubber solid content of thenatural rubber latex.
 5. The deproteinized natural rubber latexaccording to claim 2, wherein the total content of the two or moresurfactants in the aqueous dispersion medium is within a range from 0.01to 10 parts by weight based on 100 parts by weight of the rubber solidcontent dispersed in the aqueous dispersion medium.
 6. A method ofpreparing a deproteinized natural rubber latex, which comprises adding aprotease and two or more surfactants having different coagulationproperties with respect to calcium ions (Ca²⁺) to a natural rubberlatex, and maturing the natural rubber latex, wherein the two or moreanionic surfactants are stably dispersed when the concentration of Ca²⁺in an aqueous solution at 25° C. containing the surfactants is 0.1 mol/Lor less, and are coagulated when the concentration of Ca²⁺ in theaqueous solution at 25° C. is 1.0 mol/L or more.
 7. A method ofpreparing a natural rubber latex, which comprises subjecting a naturalrubber latex to a treatment for decomposition of a protein due to aprotease and a treatment for removal of a protein due to centrifugation,and dispersing the resulting creamy rubber solid content in an aqueousmedium, wherein the aqueous medium contains two or more anionicsurfactants having different coagulation properties to Ca²⁺ to a naturalrubber latex, and enables the surfactants to stably disperse when aliquid temperature is 25° C. and the concentration of Ca²⁺ is 0.1 mol/Lor less, and to coagulate when the liquid temperature is 25° C. and theconcentration of Ca²⁺ is 1.0 mol/L or more.
 8. A rubber product using adeproteinized natural rubber latex, which is obtained by dipping adipping mold, the surface of which is coated with an anode coagulant, inthe deproteinized natural rubber latex containing a vulcanizing agentadded therein of claim 1, vulcanizing a rubber film formed on thesurface of the dipping mold, and removing the rubber film from thedipping mold.